M.A.
Isn't it wonderful watching learning actually taking place?? Isn't it weird, too? Yes, it is! My younger son (2 1/2 years) does this a lot more than his brother ever did. I chalk it up to experimental learning -- what will happen if I...? He learns gravity and momentum. He also needs to learn not to do it if you want to have any windows intact! So, yes, my son is learning, but he also knows to ask before he throws, and I say yes if it's small and soft, and let him know that we don't want to break anything. If he throws it after I say "no," then I take it from him and place it out of reach for a time. If it gets out of hand, you could enforce a no throwing but outside rule. He'll learn that, too!
M.